Triaxial balun for broadband push-pull power amplifier



March 31, 19 70 I KAM 3,504,306

TRIAXIAL BALUN FOR BROADBAND PUSH-PULL POWER AMPLIFIER Filed June 2, 1969 762 70 7,? INPUT 20 I "Isl OUTPUT 3 R 1%.1. 1 14 .2. 17 4 7. 3. fly. 4.

INPUT A LOAD INVENTOR.

G'eoryefl'. Jfom AGENT.

United States Patent 3,504,306 TRIAXIAL BALUN FOR BROADBAND PUSH-PULL POWER AMPLIFIER George H. Kam, Tonawauda, N.Y., assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed June 2, 1969, Ser. No. 829,688

Int. Cl. H011) 1/00 US. Cl. 333-26 6 Claims ABSTRACT OF THE DISCLOSURE A transformer packaging arrangement comprising the combination of a reversing transformer and a step-up transformer, both of which are coaxial line sections, into a single triaxial section cup core assembly.

BACKGROUND OF THE INVENTION approach obviates the need for high frequency peaking or tuning, thereby substantially reducing the phase shift problems of push-pull operation. Further, with conventional matching networks, impedance is optimized at the high frequency end of the band, thereby resulting in a definite roll-off at lower frequencies due to mismatch. especially if the bandwidth is in excess of one octave. The use of transmission line transformers or baluns on the other hand provides a much more optimum match across the band.

Some typical transmission line applications and conventional packaging arrangements are shown in FIGS. l-4. The pictorial schematic of FIG. 1 shows a half-balun coaxial line device that may be used as an impedance transformer at the output of a single stage power amplifier to obtain a 2:1 step-up in voltage and a 4:1 step-up in impedance. The corresponding transformer schematic is shown in FIG. 2. The coaxial line of FIG. 1 includes a center conductor having an input end connected to the output of an amplifier stage (not shown), an output connected to a load, represented by resistor 12, and an outer conductor 14 connected to ground at the input end of the coaxial line. A short jumper lead 16 connects the input end of center conductor 10 to the output end of outer conductor 14. The requisite isolation between the input and output ends of outer conductor 14 is provided by looping the coaxial line within a ferrite cup core (not shown in the schematic). The use of a jumper lead 16, instead of a second coaxial line, is permitted by using a coaxial line which is significantly shorter than a quarter wavelength so as to make the propagation delay insignificant.

To provide the desired 4:1 impedance transformation, a coaxial cable is selected which has the appropriate characteristic impedance, as determined by the spacing between the center conductor 10 and outer conductor 14 and the dielectric constant of the insulation therebetween. For example, if the input impedance is 12.5 ohms and the output impedance of load 12 is 50 ohms, a coaxial cable having a ohm characteristic impedance would be selected. A step-up impedance transformer of this type is particularly useful and practical at frequencies below 100 mHz.

A conventional transmission line reversing transformer Patented Mar. 31, 1970 is illustrated by the pictorial schematic of FIG. 3, with the corresponding transformer schematic being shown in FIG. 4. In this instance, the center conductor of the coaxial line is designated by the numeral 18, and the outer conductor is identified by the numeral 20. In order to provide a polarity reversal between the input and output terminations of this coaxial section, the input signal is connected to outer conductor 20 at one end of the coaxial line while the output signal is obtained from the center conductor 18 at the opposite end of the line, center conductor 18 being grounded at the input end of the line and outer conductor 20 being grounded at the output end. In like manner to the impedance transformer of FIG. 1, the opposite ends of the reversing transformer outer conductor 20 may be isolated from each other by looping the coaxial line within a cup core device. To ensure an inherently broadband device, the characteristic impedance of the coaxial line is selected to be equal to the terminating impedances.

In order to provide proper phasing and impedance matching at the output of a broadband push-pull amplifier, the reversing section of FIG. 3 can be serially connected with the impedance transformer of FIG. 1 as illustrated in FIG. 5. The output of one of the two transistors, or vacuum tubes, comprising the push-pull amplifier is connected to one terminal of the reversing transformer outer conductor 20, as represented by input line A. The output of the other transistor, represented by input line B, is connected to the input terminal of the impedance transformer, represented by the junction of center conductors 10 and 18 and jumper leads 16. In this manner, a voltage signal applied at input A is phase reversed 180 for in-phase summing with the signal applied at input B, and the resulting combined signal is matched with the impedance of load 12 by the step-up transformer section. The serial combination of FIG. 5 is adaptable to provide input matching and phasing for a push-pull amplifier by using the loadconnected terminal of center conductor 10 as the input and obtaining the outputs from the terminal to which inputs A and B are connected. conventionally, the resulting physical packaging arrangement for this transformer combination comprises two serially connected coaxial line cup core assemblies.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved packaging arrangement for transmission line transformer combinations.

It is another object of the invention to provide a more compact and efiicient transmission line transformer packaging arrangement for providing proper phasing and impedance matching at the input or output of a broadband push-pull power amplifier.

Briefly, these objects are attained by a transmission line transformer packaging arrangement comprising a cable having center, inner and outer conductors. The inner conductor is separated from the center conductor by insulation, and the outer conductor is separated from the inner conductor by insulation. One end of the cable is adapted to receive an input signal, and the other end of the cable is adapted to provide an output signal. The terminal of the center conductor at one end of the cable is connected to the terminal of the inner conductor at the other end of the cable, whereby the inner conductor provides the dual function of a coaxial line outer conductor with respect to said center conductor and a coaxial line center conductor with respect to said outer conductor.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which: FIG. 1 is a pictorial schematic of a coaxial line imped- I 3 ance transformer to which previous reference has been made;

FIG. 2 is a transformer schematic of the device shown in FIG. 1, also referred to earlier;

FIG. 3 is a pictorial schematic of a coaxial line reversing transformer to which previous reference has been made;

FIG. 4 is a transformer schematic of the device shown in FIG. 3, also referred to earlier;

FIG. 5 is a pictorial schematic of a serial combination of the coaxial line sections shown in FIGS. 1 and 3 adapted for connection at the output of a push-pull amplifier, this figure also having been referred to earlier;

FIG. 6 is a pictorial schematic of a triaxial cable section embodying the reversing and impedance transformer functions of FIG. 5 in accordance with the invention;

FIG. 7 is a transformer schematic corresponding to the device shown in FIG. 6;

FIG. 8 is an exploded perspective view of a cup core assembly including the triaxial cable section of FIG. 6 in the form of a coil; and

FIG. 9 is an enlarged cross section of the triaxial cable.

DESCRIPTION OF PREFERRED EMBODIMENT For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

The present invention comprises the use of a triaxial cable to provide the functions of two coaxial line sections. More particularly, the reversing and impedance transformer functions of FIG. 5 are embodied in a single triaxial cable cup core assembly, thereby providing a much more compact packaging arrangement and significantly reducing power losses.

FIG. 6 is a pictorial schematic showing a section of triaxial cable 22 connected in accordance with the invention to provide the reversing and impedance transformer functions of FIG. 5. Triaxial cable 22 comprises a center conductor 24, an inner conductor 26 surrounding the center conductor, and an outer conductor 28 surrounding the inner conductor. Insulation is disposed between the center and inner conductors and between the inner and outer conductors.

As will become clear from the following description, the terminations of the three conductors comprising triaxial cable 22 are connected in a manner whereby outer conductor 28 corresponds to outer conductor 20 of the coaxial line reversing transformer, center conductor 24 corresponds to center conductor 10 of the coaxial line impedance transformer, and inner conductor 26 provides a dual function by corresponding to both center conductor 18 of the reversing transformer and outer conductor 14 of the impedance transformer. Accordingly, one end of outer conductor 28 is adapted as a signal terminal 1 to which input line A is connected, and a terminal 2 at the opposite end of outer conductor 28 is connected to ground. The terminal of inner conductor 26 at the input end of the triaxial cable, designated by the numeral 3, is also connected to ground. Terminal 4 at the opposite end of the inner conductor is connected back to terminal 5 of center conductor 24 at the input end of the cable. Center conductor terminal 5 is also adapted as a signal terminal to which input line B is attached, and the connection between terminals 4 and 5 corresponds to jumper lead 16 of the coaxial line impedance transformer. At the opposite end of the triaxial cable from that adapted to receive input signals, center conductor 24 has a terminal 6 adapted to provide an output signal to the load, represented by the resistor 12 connected between terminal 6 and ground. The transformer schematic corresponding to the FIG. 6 arrangement is shown in FIG. 7.

In order to provide a triaxial line with insignificant propagation delay, the length of cable 22 should be short- .4 er than a quarter wavelength of the intended operating frequency. The requisite isolation between the opposite terminals 1 and 2 of outer conductor 28 and between the opposite terminals 3 and 4 of inner conductor 26 is provided by looping the triaxial cable to form a coil and enclosing the coil in a cup core, as shown in FIG. 8.

A suitable flexible cable construction for this application is illustrated by the enlarged cross section of triaxial cable 22 shown in FIG. 9. A layer of insulating material 30 is wrapped or molded about a wire comprising center conductor 24, and a conductive braid enclosing the insulation 30 comprises the inner conductor 26. Outer conductor 28 comprises a second conductive braid, which is separated from the inner conducting braid 26 by a layer of insulating material 32, the outer braid 28 is then enclosed by a covering layer of insulation 34. As the transmission line comprising the inner and outer conductors is connected to provide the function of the reversing transformer of FIG. 3, the spacing between conductive braids 26 and 28 and the dielectric constant of insulation 32 are selected to provide a characteristic impedance for that transmission line which is approximately equal to the impedance of input A. The spacing between center conductor-24 and braid 26 and the dielectric constant of insulation '30, therefore, are selected to provide a characteristic impedance for the transmission line comprising conductors 24 and 26 which is approximately twice the characteristic impedance of the transmission line formed by inner conductor 26 and outer conductor 28. In this manner, the transmission line formed by the center and inner conductors will provide the step-up transformer function (FIG. 1) for which it is connected.

As illustrated in FIG. 8, the coil formed by cable 22 requires only about two turns within the cup core to provide' the requisite isolation. The cup core is composed of a ferr'ite material and has two identical halves 36 and 38. Each of the half cup cores has an outside wall 40 having a pair of diametrically opposite slots 42 and 44, a base portion 46, and a cylindrical projection 48 having an axial hole 5 0 therethrough. The coil of triaxial cable 22 is fitted within cup core half 38 so that the bottom of the coil rests on a ferrite base 46, the sides of the coil are partially enclosed by ferrite wall 40, the ferrite cylindrical projection 48 passes through the center of the coil, and the cable ends are passed through slot 42 so that the stripped portions are accessible outside the cup core. The top half 36 of the cup core is then assembled to the bottom half 38 to fully enclose the coil of triaxial cable, except for slots 42 and 44, by means of a screw 52 passing through the axial holes 50 and secured by a nut 54.

Cup cores suitable for this application are available from various sources; for example, Ferroxcube Corp. of America, Saugerties, N.Y., and Magnetics Inc., Butler, Pa.

The physical terminations and connections of the transmission line transformer are also illustrated in FIG. 8 by the use of reference numerals corresponding to those employed in FIG. 6. Hence, terminal 1 of conductive braid 28 and terminals 5 and 6 of center conductor 24 are adapted as signal terminals. Terminal 2 of conductive braid 28 and terminal 3 of conductive braid 26 are joined together and adapted to be connected to ground. Terminal 4 pf conductive braid 26 is connected to terminal 5 of center conductor 24 to provide the function of impedance transformer jumper wire 16. If the transformer of FIG. 8 is to be employed as a push-pull output device, as in FIGI 6, conductive braid terminal 1 and the junction of terminals 4 and 5 are connected to the respective pushpu'll amplifier outputs, while center conductor terminal 6 is cbnnected to the load. On the other hand, if the transformer were to be employed as an input device, terminal 6 would be the transformer input and terminal 1 and the junction of terminals 4 and 5 would provide the transformer outputs. More specifically, as an input device, terminal 1 and the junction of terminals 4 and 5 would be connected to the respective inputs of the push-pull amplifier, and terminal 6 would be connected to the output of a drive amplifier.

In summary, the present invention embodies the functions of two coaxial line transformers into a single triaxial line section. In part, this feature is accomplished by connecting the triaxial line inner conductor to provide the dual function of a coaxial line outer conductor with respect to the triaxial center conductor and a coaxial line center conductor with respect to the triaxial outer conductor. As a result, the number of cup core assemblies required to provide the dual transformer function is reduced to one, thereby reducing power losses by one-half and minimizing the circuit layout space required. In the case of high power solid-state electronic equipment employing a plurality of push-pull amplifiers, the above noted advantages become increasingly significant.

It is contemplated that the present invention may also be embodied in a packaging arrangement including a toroid in lieu of a cup core enclosure. Further, the threeconductor cable may be embodied in various strip transmission line configurations.

What is claimed is:

1. A transmission line transformer packaging arrangement comprising, in combination, a cable including first, second and third conductors, and insulation disposed between said first and second conductors and between said second and third conductors, one end of said cable being adapted to receive an input signal, the other end of said cable being adapted to provide an output signal, and the terminal of said first conductor at a first end of said cable being connected to the terminal of said second conductor at the second end of said cable whereby said second conductor is a common component of two transmission lines, one of which comprises said first and second conductors and the other of which comprises said second and third conductors.

2. A transformer in accordance with claim 1 wherein said first conductor is a center conductor, said second conductor is an inner conductor surrounding said center conductor, said third conductor is an outer conductor surrounding said inner conductor, and said inner conductor is operative to provide the dual function of a coaxial line outer conductor with respect to said center conductor and a coaxial line center conductor with respect to said outer conductor.

3. A transformer in accordance with claim 2 wherein the terminals of said center conductor at both ends of said cable are adapted as signal terminals, the terminal of said outer conductor at said first end of said cable is adapted as a signal terminal, and the terminal of said inner conductor at said first end of said cable and the terminal of said outer conductor at said second end of said cable are adapted to be connected to ground.

4. A transformer in accordance with claim 3 wherein the spacing between said inner and outer conductors and the dielectric constant of said insulation therebetween are selected to provide a first characteristic impedance for the transmission line comprising said inner and outer conductors, and the spacing between said center and inner conductors and the dielectric constant of said insulation therebetween are selected to provide a second characteristic impedance for the transmission line comprising said center and inner conductors which is approximately twice said first characteristic impedance.

5. A transformer in accordance with claim 4 wherein said cable is arranged to form a coil, and further including a ferrite cup core enclosing the coil formed by said cable.

6. A transformer in accordance with claim 5 wherein said cable is shorter than a quarter wavelength of the intended operating frequency.

References Cited UNITED STATES PATENTS 2,581,156 1/1952 Weighton 333-26 X 2,966,640 12/19 0 Eiland 33326 HERMAN KARL SAALBACH, Primary Examiner MARVIN NUSSBAUM, Assistant Examiner US. Cl. X.R. 333-97 

